1. Field of the Invention
The present invention relates to Hard Disk Drives (HDDs). More particularly, the present invention relates to a technique for controlling the spacing between the pole tips of a write element and a magnetic medium (i.e., the flying height of a slider) of an HDD in a linear manner using a control signal.
2. Description of the Related Art
FIG. 1 shows an exemplary hard disk drive (HDD) 100 having a servo system for positioning a slider assembly 101 over a selected data information track on a magnetic disk 102 for writing data to and/or reading data from the selected track. The servo system of HDD 100 includes an actuator 104, such as a rotary voice-coil motor (VCM), for positioning an actuator arm 105, a read/write head suspension 106 and slider assembly 101 over a selected track. Slider assembly 101 includes a read/write head (not shown in FIG. 1) having a read element, such as a Giant Magnetoresistive (GMR) element, and a write element that respectively read data from and write data to a selected data track. While HDD 100 shows only a single magnetic disk 102, HDDs typically have a plurality of stacked, commonly rotated rigid magnetic disks and a corresponding number of actuator arms, read/write head suspensions and slider assemblies.
FIG. 2 depicts a cross-sectional side view of an exemplary read/write head 200 that is part of slider assembly 101. Read/write head 200 includes a thin film structure consisting of a stacked read element 201 and a write element 202 that are situated at the trailing edge of a slider body 203. Read element 201 is positioned in a read gap 204 between a bottom shield 205 and a bottom yoke/top shield 206. Write element 202 is formed by bottom yoke/top shield 206, top yoke 207, coil windings 208 of a write coil and write gap 209.
At the beginning of a write operation, write element 202 heats up due to power dissipation caused by a write current through the write coil of the write element and by the eddy current losses in bottom and top yokes 206 and 207 of write element 202. The warming causes pole tips 210 and 211 of write element 202 to protrude from the slider body. The pole-tip protrusion is proportional to the power dissipation in write element 202 and has a step response that is exponential as a function of time with a (thermal) time constant of several hundreds of microseconds. The clearance 212 between pole tips 210 and 211 and the magnetic recording medium 213, commonly referred to as the flying height of the slider, is reduced linearly as a function of the power dissipation. Accordingly, the write performance can change dramatically due to the change in effective flying height.
One conventional approach for decreasing the protrusion of the pole tips is by improving the heat sinking of the write coil, such as disclosed by U.S. Patent Application No. 2003/0039067 A1. Improving the heat sinking, however, becomes increasingly difficult as the dimensions of the write element decrease and as the data rate increases. Additionally, as the dimensions of the write element decrease, the area for the heat sinking also decreases. As the data rate increases and write current reversal times decrease, the power dissipation in the write element goes up due the increased eddy current losses in the yoke. See, for example, K. B. Klaassen et al., “Nanosecond and sub-nanosecond writing experiments,” IEEE Transactions on Magnetics, Vol. 35, pp. 625–631, March 1999.
Another conventional approach for controlling the protrusion of the pole tips is to keep the protrusion of the write element unchanged going from a read to a write operation by keeping the temperature of the write element constant. A separate heater element 214 (FIG. 2) in the form of a resistive film has been provided with the write head and has been electrically excited with a voltage or current.
Yet another conventional approach for controlling pole-tip protrusion is to inject a “thermal protrusion control current” through a center-tap of the write-head coil. The current divides evenly between the two halves of the write-head coil and produces no net magnetic excitation of the write head yoke.
Nevertheless, what is needed is a technique for controlling pole-tip protrusion by controlling the spacing between the pole tips of a write element and the magnetic medium in a linear manner using a control signal.